Re: [PATCH] mm/memcontrol: avoid false sharing between vmstats and events

[Michal Hocko]

On Mon 17-08-26 03:38:35, Usama Arif wrote:
> Moving v1 userspace eventfd handling into memcontrol-v1.c shrank
> struct vmpressure from 112 to 24 bytes when CONFIG_MEMCG_V1 is disabled.
> This moved memory_events_local[MEMCG_SWAP_FAIL] and the hot
> vmstats_percpu pointer onto the same cacheline.
> 
> The stress-ng mremap stressor exercises MADV_PAGEOUT with swap
> disabled, generating about 20 million MEMCG_SWAP_FAIL updates per
> 60-second run on a 176-CPU test system.  Those writes bounce the line
> while memcg statistics paths load vmstats_percpu.
> 
> Move cgwb_list into the existing alignment gap and cacheline-align
> vmstats_percpu.  This separates the pointer from the event counters
> without increasing the size of struct mem_cgroup in the tested
> configuration.
> 
> The blamed commit reduced median mremap throughput by 4.38% on the
> test system with one socket. The patched kernel brings the performance
> to within 0.5% of the parent which is within the observed boot-to-boot
> spread (up to 1.2%).
> 
> Fixes: ea928e9e18da ("mm/vmpressure: move v1 userspace eventfd code into memcontrol-v1.c")
> Reported-by: kernel test robot <yi1.lai@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202608131743.c6a7dda4-lkp@xxxxxxxxx
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!

> ---
>  include/linux/memcontrol.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e78bc98ab229b..215e2e87f42b2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -268,10 +268,15 @@ struct mem_cgroup {
>  #endif
>  	int kmemcg_id;
>  
> -	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
> -
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct list_head cgwb_list;
> +#endif
> +
> +	/* Keep the hot per-CPU stats pointer away from memory event counters. */
> +	struct memcg_vmstats_percpu __percpu *vmstats_percpu
> +		____cacheline_aligned_in_smp;
> +
> +#ifdef CONFIG_CGROUP_WRITEBACK
>  	struct wb_domain cgwb_domain;
>  	struct memcg_cgwb_frn cgwb_frn[MEMCG_CGWB_FRN_CNT];
>  #endif
> -- 
> 2.53.0-Meta

-- 
Michal Hocko
SUSE Labs